1. Field of the Invention
The present invention generally relates to the formation of apertures in sheet material by punching and, more particularly, to the monitoring and control of punching tools operated at high speed, especially as used in the fabrication of electronic circuit components.
2. Description of the Prior Art
The manufacture of many articles involves the formation of apertures therein. In particular, it has been the common practice in the construction of electrical and electronic devices to mount components on a perforated, insulating board or other substrate by passing leads through apertures in the board. In such a case, connections are often formed by a conductive metal pattern on the board and also by wiring passed through the board, as in constructions involving the well-known printed circuit.
In recent years, electronic circuits and devices of greatly increased complexity have been fabricated with multiple layers. One such type of device is the multi-layer module (MLM) which employs a large number of insulative layers to separate numerous conductive patterns. The respective conductive patterns may be selectively connected at holes formed in the insulative layers, commonly referred to as vias.
A particular form of MLM is the multi-layer ceramic (MLC) device in which the insulative layers are initially formed of an uncured ceramic, referred to as a green sheet. Conductive patterns are formed thereon by screening of a conductive paste onto a surface of the green sheet with a stencil or mask. Vias may be filled in the same manner or even in the same screening operation. After the conductive patterns are formed on the respective green sheets, the green sheets are assembled into an accurately registered stack and sintered under pressure to cure the ceramic and form the complete interconnection arrangement embedded therein.
Of course, in such structures, the vias are extremely small and very numerous in each of the 20 to 100 or more green sheets which may be present in a single device. This is particularly true for so-called distribution layers which serve to change the nominal conductive feature spacing in the so-called connection layers to a size and spacing (e.g. pitch) compatible with integrated circuits which are mounted thereon. Therefore, the apertures which form the vias must be accurately positioned and must be of very accurate geometry. To achieve this accuracy, a punch is generally used to form the vias. A punch is also desirable because the material from the punched hole will be of a relatively large particle size in comparison with other methods which cut or erode material from the aperture. Such material can be a source of contamination of the green sheet during further processing and larger particle size facilitates control thereof.
A punch apparatus must be very large in comparison to the holes formed in order to develop the force necessary to successfully and cleanly punch through the material (e.g. green sheet). The machining of punches to form multiple holes in a single stroke is more expensive than single hole punches. Further, the results obtained from multiple hole punches are generally inferior to the hole quality obtained with a single hole punch in terms of geometric accuracy and positioning accuracy. However, if a single hole punch is used, the number of holes which must be formed in each green sheet dictates that the punch apparatus be operated a very high speed in order to achieve acceptable throughput during manufacturing processes.
Since such punches are mechanical devices, although generally operated by electrical systems, accuracy may be compromised at higher speeds where mechanical limitations are encountered due to high acceleration and deceleration forces and the geometry of the punch mechanism. In particular, when the velocity of the punch is increased, the reciprocating portion of the punch may strike other portions of the punch structure at the ends of the punching stroke and elastically rebound therefrom. Such rebounding is essentially an uncontrolled motion of the punch and, if severe, may cause unintended contact between the punch and the workpiece. At the high accuracies required in MLC structures, such contact will almost invariably cause significant damage to the green sheet as dimples in the green sheet surface which will affect the manufacturing yield of the devices made.
Since the length of the punch stroke must be limited for use at high punching rates and accuracies, uncontrolled motion due to rebounding is a substantial limiting factor in regard to the punching rates obtainable. Compensation or avoidance of such effects are non-trivial. While mechanical damping is somewhat effective in reducing rebounding, in practice, it merely allows increase in speed until deleterious effects of the rebounding motion are again encountered. Such mechanical damping often does not produce substantial gain in punching rates and the extent to which it can be applied is limited since mechanical damping causes significant punch speed reduction during the return stroke and thus increases the total time duration of a single punching operation.